[Bug] Add a Rescan method to Manager interface V2: added log, double '_' fixed in mediaconsole and reverted useless assignement

- Resolves bug #4
- Make use of new API gupnp_context_manager_rescan_control_points()
- Documentation updated.

Signed-off-by: Sébastien Bianti <sebastien.bianti@linux.intel.com>